Operation of heads column

ABSTRACT

A process for the enhanced recovery and operation of hydrogen cyanide (HCN)/heads column obtained from the reactor effluent of an ammoxidation reaction of propane, propylene or isobutylene by reducing the polymer formation above the feed tray in the heads tower.

SUMMARY

[0001] The present invention is directed to an improved process for themanufacture of acrylonitrile or methacrylonitrile. In particular, thepresent invention is directed to improved operation of the heads, or HCNseparation, column in the acrylonitrile and methacrylonitrile recoveryprocess. Applicant has discovered a previously unknown relationshipbetween the formation of undesirable polymeric HCN in the heads columnand the formation of an aqueous second liquid phase in the heads columnabove the feed tray. The present invention is directed towardspreventing the formation of the aqueous phase in the heads column abovethe feed tray, since the presence of this aqueous phase causes theformation of unwanted and detrimental polymeric HCN. Previous art wasdirected at reducing the pressure of the heads tower, resulting in loweroperating temperatures and perceived reduction in the polymerizationrates of HCN. The instant invention is directed at disrupting themechanism of the HCN polymerization, which occurs as ionicpolymerization in the aqueous phase. By practicing the presentinvention, unwanted polymerization of HCN may be reduced, fouling of theheads column may be greatly diminished or eliminated, and increasedproduction of desirable products may be achieved.

FIELD OF THE INVENTION

[0002] The present invention is directed to an improved process for themanufacture of acrylonitrile or methacrylonitrile. In particular, thepresent invention is directed to the improvement in the recovery andoperation of hydrogen cyanide separation column utilized during themanufacture of acrylonitrile or methacrylonitrile.

[0003] Recovery of acrylonitrile/methacrylonitrile produced by theammoxidation of propane, propylene or isobutylene on a commercial scalehas been accomplished by quenching the reactor effluent with waterfollowed by passing the gaseous stream containing acrylonitrile ormethacrylonitrile, as well as byproduct HCN, resulting from the quenchto an absorber where water and the gases are contacted in countercurrentflow to remove substantially all the acrylonitrile or methacrylonitrile.The aqueous stream containing HCN and the acrylonitrile ormethacrylonitrile is then passed through a series of distillationcolumns and associated decanters for separation and purification ofproduct acrylonitrile or methacrylonitrile from a vapor streamcontaining substantially all the HCN.

[0004] Typical recovery and purification systems that are used duringthe manufacture of acrylonitrile or methacrylonitrile are disclosed inU.S. Pat. Nos. 4,234,510 and 3,885,928, assigned to the assignee of thepresent invention and herein incorporated by reference.

SUMMARY OF THE INVENTION

[0005] It is the primary object of the present invention to provide animproved process for the recovery and operation of byproduct HCN in themanufacture of acrylonitrile or methacrylonitrile.

[0006] Another object of the present invention is to provide an improvedprocess for the recovery of acrylonitrile, methacrylonitrile, or HCNobtained from the reactor effluent of an ammoxidation reaction ofpropane, propylene or isobutylene comprising passing the reactoreffluent through an absorber column, a recovery column and a headscolumn wherein the improvement comprises operating the heads column in amanner which inhibits the formation of an aqueous phase above the feedtray of the heads column.

[0007] An additional object of the present invention is to provide animproved process for the recovery of acrylonitrile, methacrylonitrile,or HCN obtained from the reactor effluent of an ammoxidation reaction ofpropane, propylene or isobutylene by operating the heads column in amanner which inhibits the formation of an aqueous phase above the feedtray of the heads column, such as increasing reflux ratios; using a sidedecanter to split and remove the aqueous phase from the column; using acooler feed stream to increase the stripping in the column; increasingthe number of stripping trays; using an intermediate condenser above thefeed to supplement the overhead condenser; subcooling the reflux stream;increasing reboiler and overhead condenser duties to increase refluxflow rates; control operating pressure to shift the equilibrium betweenthe two liquid phases; and other methods known to those skilled in theart that would increase reboiler duty, and the associated strippingeffectiveness of the heads column. Increasing the hydrogen cyanidereflux or concentration of hydrogen cyanide above the feed tray can alsobe achieved through higher HCN production levels for eliminating thesecond liquid phase. Any increased tray efficiency also allows morestripping effectiveness and is helpful in eliminating the undesiredsecond liquid phase.

[0008] Yet another object of the present invention is to provide animproved process for the recovery of acrylonitrile, methacrylonitrile,or HCN obtained from the reactor effluent of an ammoxidation reaction ofpropane, propylene or isobutylene comprising passing the reactoreffluent through an absorber column, a recovery column and a headscolumn wherein the improvement comprises feeding extra HCN to the headscolumn, either by operating the ammoxidation reactor in a manner toproduce a higher concentration of HCN to other products, or by recyclingHCN to the heads column, to permit operation of the heads column in amanner that reduces or eliminates the formation of the undesirableaqueous phase.

[0009] Additional objects, advantages and novel features of theinvention will be set forth in part in the description which follows,and in part, will become apparent to those skilled in the art uponexamination of the following or may be learned by the practice of theinvention. The objects and advantages of the invention may be realizedand obtained by means of the instrumentalities and combinationsparticularly pointed out in the appended claims. To achieve theforegoing and other objects and in accordance with the purpose of thepresent invention as embodied and broadly described herein, the processof the present invention comprises transporting the reactor effluentobtained during the ammoxidation of propane, propylene or isobutylene toa quench column wherein the hot effluent gases are cooled by contactwith an aqueous spray, passing the cooled reactor effluent overhead toan absorber column wherein the HCN and crude acrylonitrile ormethacrylonitrile is absorbed in water, passing the aqueous solutioncontaining the HCN and acrylonitrile or methacrylonitrile, plus otherimpurities to a first distillation column (recovery column), where asignificant portion of the water and impurities are removed as a liquidbottoms product, while HCN, water, a minor portion of impurities andacrylonitrile or methacrylonitrile are removed as an overhead vaporstream. This overhead vapor stream is further cooled using a heatexchanger, and directed to adecanter, to separate and condensed liquidswhich are returned to the recovery process, while the remaining vaporstream is directed to a flare, incinerator, or other disposal process.The organic stream is fed to the heads column for separation of HCN fromacrylonitrile.

[0010] In a preferred embodiment of the present invention, the processis performed with the reactor effluent obtained from the ammoxidation ofpropane or propylene, ammonia and oxygen to produce acrylonitrile.

[0011] In a still preferred embodiment of the present invention, thereactor effluent is obtained by the reaction of propane, propylene,ammonia and air in a fluid bed reactor while in contact with a fluid bedcatalyst. Conventional fluid bed ammoxidation catalyst may be utilizedin the practice of the invention. For example, fluid bed catalyst asdescribed in U.S. Pat. Nos. 3,642,930 and 5,093,299, herein incorporatedby reference, may be utilized in the practice of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

[0012]FIG. 1 is a schematic representation of the process as it appliesto the manufacture of acrylonitrile and improved recovery and operationof HCN separation column.

DETAILED DESCRIPTION OF THE INVENTION

[0013] The general recovery and purification of acrylonitrile ofmethacrylonitrile, and the present invention will now be described indetail with reference to FIG. 1. The reactor effluent 11 obtained by theammoxidation of propane, propylene or isobutylene, ammonia and oxygencontaining gas in a fluid bed reactor (not shown) while in contact witha fluid bed ammoxidation catalyst is transported to a quench column 10via transfer line 11, wherein the hot effluent gases are cooled bycontact with water spray, 14. The cooled effluent gas containing thedesired product (acrylonitrile or methacrylonitrile, acetonitrile andHCN) is then passed into the bottom of an absorber column 20 via line 12wherein the products are absorbed in water which enters absorber column20 from the top via line 24. The non-absorbed gases pass from theabsorber through pipe 22 located at the top of the absorber 20. Theaqueous stream containing the desired product is then passed via line 23from the bottom of absorber 20 to the upper portion of a firstdistillation column 30 (recovery column) for further productpurification. The product is recovered from the top portion of recoverycolumn 30 and sent to a second distillation column 40 (heads column) 40via line 32, while water and other impurities are removed from therecovery column 30 via line 33. In the heads column 40, the HCN is takenoverhead and removed from the column via line 42, cooled in overheadcondenser 80, and the resulting material directed to reflux drum 50 vialine 41. Liquid reflux from the reflux drum 50 is returned to the upperportion of the heads tower via line 53. Vapor phase material is removedfrom the reflux drum 50 via line 52 and cooled in HCN product condenser90.

[0014] A significant operational problem experienced in the recovery andpurification of products in the acrylonitrile and methacrylonitrileproduction process is the formation of polymeric HCN in the headscolumn, sometimes also known as the HCN column (40). In particular,polymeric HCN forms on the trays and internals in the heads column abovethe column feed location (where line 32 enters column 40). The solid,polymeric HCN fouls distillation trays, over flow weirs, downcomers, andthe like, as well as disrupting the hydraulic balance of theliquid/vapor interfaces in the heads column. The polymerizationincreases the column pressure drop, and the corresponding increasedtemperatures in the column further increases the polymer formation. Thispolymer eventually requires a costly and time consuming shutdown of thepurification section and a column cleaning exercise.

[0015] A less precise understanding of the phenomena led priorpractitioners to reduce the operating pressure and hence temperature ofthe heads column, thus reducing the rate of the polymerization reactionwhich forms the fouling material. Applicant has discovered that thepolymerization mechanism is dependent upon the presence of a secondliquid, namely, an aqueous phase in the heads column. This aqueous phaseprovides the conditions conducive for the ionic polymerization of HCN,to form solid polymeric HCN. The polymeric HCN precipitates to block theactive areas as well as downcomers of the distillation trays, andcoheres to other polymeric HCN to foul the tower internals. Applicant'sdiscovery of this previously unknown and unsuspected mechanism permitsapplicant to operate such distillation columns with significantly lowerrates of HCN polymerization and resulting fouling. The enhancedoperation can be effected by operating the heads column in a manner toreduce or eliminate the formation of this aqueous phase.

[0016] Since the formation of an aqueous layer was not appreciated as apotential source of HCN polymerization, there was no incentive to reducethe formation of this aqueous layer available in the current art.Techniques to reduce the formation of the aqueous layer include, but arenot limited to, increased reflux ratios; the use of a side decanter tosplit and remove the aqueous phase from the column; use of a cooler feedstream to increase the stripping effectiveness of the column; increasednumber of stripping trays; use of an intermediate condensor above thefeed to supplement the overhead condenser; subcooling the reflux stream;increased reboiler and overhead condensor duty to increase reflux flowrates; control operating pressure to shift the equilibrium between thetwo liquid phases; and methods that would increase reboiler duty, andassociated stripping effectiveness of the head column. Increasing thehydrogen cyanide reflux or concentration of hydrogen cyanide above thefeed tray can also be achieved by operating the reactor section of theprocess to produce higher weight percentages of HCN in the reactorproduct, increasing the percentage of HCN in the heads column feed,which results in the reduction or elimination of the second liquidphase. Any increased tray efficiency also allows more strippingeffectiveness and is helpful in eliminating undesirable aqueous secondliquid phase.

[0017] Preferably, the ammoxidation reaction is performed in a fluid bedreactor although other types of reactors such as transport line reactorsare envisioned. Fluid bed reactors, for the manufacture of acrylonitrileare well known in the prior art. For example, the reactor design setforth in U.S. Pat. No. 3,230,246, herein incorporated by reference, issuitable.

[0018] Conditions for the ammoxidation reaction to occur are also wellknown in the prior art as evidenced by U.S. Pat. Nos. 5,093,299;4,863,891; 4,767,878 and 4,503,001; herein incorporated by reference.Typically, the ammoxidation process is performed by contacting propane,propylene or isobutylene in the presence of ammonia and oxygen with afluid bed catalyst at an elevated temperature to produce theacrylonitrile or methacrylonitrile. Any source of oxygen may beemployed. For economic reasons, however, it is preferred to use air. Thetypical molar ratio of the oxygen to olefin in the feed should rangefrom 0.5:1 to 4:1, preferably from 1:1 to 3:1. The molar ratio ofammonia to olefin in the feed in the reaction may vary from between0.5:1 to 5:1. There is really no upper limit for the ammonia-olefinratio, but there is generally no reason to exceed a ratio of 5:1 foreconomic reasons.

[0019] The reaction is carried out at a temperature of between theranges of about 260° to 600° C., but the preferred ranges being 310° to500° C., especially preferred being 350° to 480° C. The contact time,although not critical, is generally in the range of 0.1 to 50 seconds,with preference being to a contact time of 1 to 15 seconds.

[0020] In addition to the catalyst of U.S. Pat. No. 3,642,930, othercatalysts suitable for the practice of the present invention are setforth in U.S. Pat. No. 5,093,299, herein incorporated by reference.

[0021] The conditions under which the absorber column, recovery columnand heads column are maintained range between 5 to 7 psig (80° F. to110° F.), and 1 to 4.5 psig (155° F. to 170° F.), respectively.

EXAMPLES

[0022] ASPENPLUS® process simulations of the heads column were used toidentify operating conditions which eliminated the presence of anaqueous phase on trays above the column feed location. A column feedcolumn temperature was selected and a tray efficiency specified. Thereflux ratio of the heads column was then adjusted for each case untilno aqueous third phase formed on trays above the column feed location.Acceptable product purity was defined as an overhead stream compositionof less than 50 ppm acrylonitrile, and a bottoms stream composition ofless than 100 ppm HCN.

[0023] For all examples, the heads column feed had a nominal compositionof 83 wt % acrylonitrile, 10 wt % HCN and 7 wt % water, and wasintroduced to the heads column at a rate of 40,000 lb/hr at 30 psia. Theheads column had 64 trays, a reboiler and an overhead condenser. Thetray efficiency used in these simulations was 60%. For all examples, thetrays are numbered starting from the Heads column top tray.

Example 1

[0024] The main feed was introduced to the column at tray 25. With a 100degree feed temperature, the third aqueous phase was eliminated at thereflux ratio of 4.95, while still maintaining the overhead and bottomsproducts within specification limits. All reflux ratios below 4.95formed an undesirable aqueous layer.

Example 2

[0025] The main feed location was moved up by five trays and the feedwas introduced to the column at tray 20. With a 100 degrees F. feedtemperature, the third aqueous phase was eliminated at the reflux ratioof 4.3, while still maintaining the overhead and bottoms products withinspecification limits. Thus, introducing 5 more stripping trays reducedthe reflux required for the elimination of the undesirable aqueous phasefrom 4.95 in Example 1 to 4.3 in this case.

Example 3

[0026] The main feed at 80 degrees F. was introduced to the column attray 25. With this colder feed temperature, the third aqueous phase waseliminated at the reflux ratio of 4.24 instead of 4.95 reported inExample 1, while still maintaining the overhead and bottoms productswithin specification limits.

Example 4

[0027] The primary feed was split in two portions: 75% of the feed wasintroduced on tray 20, and 25% of the feed was introduced on tray 25.The feed temperature was maintained at 100 degrees F. for both theportions. The third aqueous phase was eliminated at a reflux ratio of4.1, while still maintaining the overhead and bottoms products withinspecification limits. This compares favorably with a reflux ratiorequired of 4.95 in Example 1 or 4.3 in Example 2.

Example 5

[0028] The primary feed was maintained at 100 degrees F. and wasintroduced on stage 25. An intermediate tray condenser withdrew all theliquid phase material from tray 20, cooled the material to 60 degreesF., and returned the cooled liquid material to tray 21. The thirdaqueous phase was eliminated at a reflux ratio of 4.6, while stillmaintaining the overhead and bottoms products within specificationlimits.

Example 6

[0029] The primary feed was maintained at 100 degrees F. and wasintroduced on tray 25. A total of 400 lb/hr of 99.8 wt % HCN, maintainedat 80 degrees F., was fed to tray 20. The third aqueous phase waseliminated at the reflux ratio of 4.3, while still maintaining theoverhead and bottoms products within specification limits. One shouldnote here that the pure HCN addition can be made anywhere in the sectionbetween the Heads column top tray and the feed tray. Operation of anindustrial facility with reactor conversion tuned to produce higherpercentages of HCN in the reactor product stream, resulting in a higherpercentage of HCN in the heads column feed streams would have resultssimilar to this example.

Example 7

[0030] The primary feed was maintained at 100 degrees F. and wasintroduced on tray 25. A two-phase, side-decanter, which may be operatedat sub-ambient temperatures, withdrew all the liquid phase material fromtray 24, decanted the aqueous phase from the organic phase, and returnedthe organic phase material to tray 25. The third aqueous phase waseliminated at a reflux ratio of 4.8, while still maintaining theoverhead and bottoms products within specification limits.

[0031] It should be further noted that combinations of various ideas tosubstantially or completely eliminate the formation of the undesirableaqueous phase can yield an optimum solution which would be determined byspecific constraints. As will be evident to those skilled in the art,various modifications of this invention can be made or followed in lightof the foregoing disclosure and discussion without departing from thespirit and scope of the disclosure or from the scope of the claims.

What is claimed is:
 1. A process for the recovery of acrylonitrile,methacrylonitrile or hydrogen cyanide obtained from the reactor effluentof an ammoxidation reaction of propane, propylene or isobutylenecomprising passing said reactor effluent through an absorber column, arecovery column and a heads column wherein the improvement comprisesoperating said head column in a manner which inhibits the formation ofan aqueous phase above the feed tray of said heads column.
 2. Theprocess of claim 1, wherein said operating manner of said heads columncomprises increasing the reflux ratio of said heads column to the pointthat no aqueous phase forms above the feed tray.
 3. The process of claim2, wherein said operating manner of said heads column comprises feedingmore hydrogen cyanide to said heads column to achieve conditionsequivalent to higher reflux ratio.
 4. The process of claim 3, whereinsaid feeding results from recycling purified HCN to the heads column. 5.The process of claim 3, wherein said feeding results from operating anammoxidation reactor in a manner to produce said reactor effluent withhigh concentrations of HCN.
 6. The process of claim 1, wherein saidoperating manner of said heads column comprises using a side decanter toremove aqueous material from said heads column.
 7. The process of claim1, wherein said operating manner of said heads column comprisesincreasing the number of stripping trays of said head column.
 8. Theprocess of claim 1, wherein said operating manner of said heads columncomprises increasing the reboiler duty of said heads column.
 9. Theprocess of claim 1, wherein said operating manner of said heads columncomprises using an intermediate condenser above the feed tray of saidheads column and below the reflux condenser of said heads column. 10.The process of claim 1, wherein said operating manner of said headscolumn comprises cooling the feed stream to said heads column to atemperature that no aqueous phase forms above the feed tray.
 11. Theprocess of claim 1, wherein said operating manner of said heads columncomprises subcooling the reflux stream to said heads column.
 12. Theprocess of claim 1, wherein said operating manner of said heads columncomprises reducing operating pressure of said heads column with orwithout other changes so that the second liquid phase region is reducedwith pressure reduction as well.
 13. Acrylontrile, methacrylonitrile orhydrogen cyanide produced using the process of claim 1.